Lithography process is an essential and important procedure to a variety of electric devices, such as semiconductor devices and liquid crystal display. In general, a lithography process comprises a step of allowing lights or rays radiate through a photo mask with pre-designed patterns to a substrate coated with a photoresist layer for illuminating the photoresist layer corresponding to these patterns, then the substrate is subject to a developer solution for removing un-exposure photoresist layer to form patterns corresponding to the pre-designed patterns on the substrate. Alternatively, depending on the different material of photoresist, an exposure photoresist layer will be removed in the developer solution.
The patterns formed on the substrate by the foregoing lithography process, these patterns have the same dimensions or sizes because the environmental parameters to the whole photoresist layer are the same during the exposure process. However, for certain electric devices, the structure of the patterns with different dimensions is demanded. For this kind of electric devices, a plurality of lithography processes for different dimensions are needed. For example, when an electric device having patterns with two different dimensions, the fabricating process of the electric device requires two different lithography processes. The steps of the aforementioned fabricating process includes a steps of forming a photoresist layer with a thickness on a substrate by spin-coating; subsequently, a photo mask with patterns is placed and aligned to a corresponding location over the photoresist layer for exposure process; successively, a development process and a post-bake process are performed for forming a plurality of first patterns with a first dimension. After the first patterns are formed by this lithography process, next lithography process for forming a plurality of second patterns with a second dimension is processed. The steps of the later lithography process includes steps of forming another photoresist layer with different thickness from previous one on the substrate by spin-coating; subsequently, the photo mask with patterns is used again and is placed and aligned to a corresponding location over the later photoresist layer for second exposure process; successively, the first patterns structure and second patterns structure are constructed on the substrate after another development process and another post-bake process are performed.
These processes for forming the layers with different dimensions are almost the same. Moreover, it takes quite a long time to perform the fabricating process due to several lithography processes. Thus, the cost of production for a device with such kind of structure is relatively expensive.
In order to solve the foregoing problem, a multi tone mask utilized to reduce the steps of lithography processes is introduced. The multi tone mask is a kind of photo mask which has a plurality of patterns regions with various transmittances thereon. Referring to FIG. 1, it shows an illustration of a lithography process with a multi tone mask. The multi tone mask 100 has a first pattern region 101 with a first percentage of transmittance, and a second pattern region 103 with a second percentage of transmittance. For example, the first percentage of transmittance is 100%, and the second percentage of transmittance is X %. During the exposure process of the lithography process with the multi tone mask 100, the light will all pass through the first pattern region 101 with the 100% transmittance and only X percent of the light will pass through the second region 103 with the X % transmittance. Thus, the photoresist layer coated on a substrate 110 will produce different dimensions of patterns structures after developing process, such as large size patterns 111 and small size patterns 113 as shown in FIG. 1, due to different levels of photoreaction to the different regions of the photoresist layer on the substrate 110 according to different light intensities. Therefore, a plurality of dimension of patterns structure formed on the substrate only needs one lithography process by utilizing a multi tone mask designed a plurality of corresponding pattern regions with different transmittances when the different dimensions of the patterns structure is required.
Referring to Taiwan issue patent No. I245160, which relates to a method of forming a stamper of a light guide plate by half-tone technology, and it discloses a lithography process with a half-tone technology for forming a plurality of photoresist patterns with different highnesses on a light guiding plate after removing parts of photoresist layer; and smooth and spherical mirror surfaces of photoresist patterns is presented after a flow process. Moreover, referring to Taiwan issue patent No. I3029089, which relates to a fabrication method of active device array substrate, and it discloses five lithography processes for forming a gate, a channel layer, a source, a drain, a protective layer, and a pixel electrode on a thin-film transistor (TFT) array substrate are reduced to three lithography processes by utilizing multi tone mask. Thus, the fabricating time and cost of produce are reduced. Furthermore, referring to Taiwan publication patent application No. 200907560, which relates to an exposure process, a fabricating method of pixel structure and half tone mask thereof. It refers to a lithography process for a glass substrate with large surface causes a non-uniform dimensions problem of the photoresist patterns. This problem can be solved by utilizing a half tone mask (a kind of multi tone mask) to form uniform size of the photoresist patterns on a large surface. As mentioned above, multi tone masks are widely used in the lithography processes in the recent year.
However, multi tone masks designed several patterns regions with different transmittances thereon for fabricating the patterns with different dimensions or sizes, so that the cost of fabricating multi tone masks is very high. For example, the cost of fabricating multi tone masks is over 2˜3 times higher than the cost of fabricating traditional photo masks. Although, the multi tone mask technology may reduce the lithography processes to fabricate a device, however, the cost of fabricating the device is still high due to the high cost of the multi tone masks.
Therefore, it is required a lithography process for forming a photoresist structure having patterns with different dimensions to not only reduce the fabricating time but also the cost of the production.